This invention relates generally to optical playback apparatus and universal focusing systems, and more particularly to axicon-type focusing elements.
In the area of optical recording and playback apparatus, the accurate resolution of data on an optical data record has proven difficult to obtain. U.S. Pat. Nos. 3,501,586 and 4,090,031 to Russell and 4,142,209 to Hedlund, et al. disclose examples of such apparatus.
Typically, optical data is recorded along a track in the form of a very fine optical pattern, such as closely-spaced microscopic dots. This data is played back by scanning a light beam along the track to modulate the light beam in accordance with the optical pattern. The modulated beam is either transmitted or reflected to a light detector which produces an electrical output signal in accordance with the modulation of the beam. If all goes well, this signal faithfully reproduces the optical pattern to play back the originally recorded signal. However, numerous factors effect the ability to faithfully reproduce the recorded signal.
One important factor is the ability to focus the beam in the plane of the optical pattern and to maintain focus during playback. Prior playback systems employ a fixed or adjustable focal length focusing system including spherical or other point-focusing lenses. The focal point of the focusing system is positioned in the plane of the optical pattern by a variety of techniques. Examples of such techniques include moving either the optical elements or the record or both along the optical axis.
Initially, the optical elements and record are positioned manually. Because the optical pattern is typically very small and dense, positioning must be very precise. The problem is compounded by the need for accurate angular alignment of such elements. Thus, positioning the optical elements can be very difficult and time consuming.
Once the optical elements and record are initially positioned, focusing continues to be a problem during the operation of the playback system. The optical records are seldom truly planar. The mechanisms supporting and moving the records also have a certain amount of play in them. In the Hedlund, et al. apparatus of U.S. Pat. No. 4,142,209, the optical elements are shifted along the optical axis to scan different tracks. All of these factors can dynamically change the relative positions of the optical elements and record, causing the focal point to deviate from its ideal position. As a result, the image of data pattern tends to move in and out of focus causing errors during playback.
Accordingly, there is a need in optical playback systems for means for focusing the light beam on optical data patterns which obviates the need for highly precise positioning of the focusing elements and record. Such beam-focusing means should also be immune to dynamic variations in the relative position of its elements and the record in an optical playback apparatus.
In the field of optics, researchers have long sought a universal focus lens. In 1954, J. H. McLeod published a paper entitled "The Axicon: A New Type of Optical Element" in the Journal of the Optical Society of America, No. 44, pp. 592-597, indicating the discovery of such a lens. In this paper, McLeod defined the axicon and described several examples: a toric lens, a right conical lens and reflector, a hollow refractive sphere and a hollow reflective cylinder. McLeod also described possible uses for axicons in telescopes, microscopes, projectors and autocollimators.
Since 1954, further examples and applications of axicons have been identified. In an article entitled "Focons and Foclines as Concentrators of the Radiation of Extended Objects," Soviet Journal of Optical Technology, February 1977, pp. 66-69, V. K. Baranov describes two families of reflecting axicons--the parabolic-toroidal focon and the parabolic-cylindrical focline--and their ability to concentrate light from an extended source to a point. M. Rioux, et al. discussed the use of axicons in combination with lasers in an article entitled "Linear, Annular, and Radial Focusing with Axicons and Applications to Laser Machining," Applied Optics, Vol. 17, No. 10, pp. 1532-1536, May 15, 1978.
Axicons have also been found to be applicable to forms of energy other than electromagnetic. Acoustical axicons and their applications are discussed in two articles by C. B. Burckhardt, et al. entitled "Ultrasound Axicon: A Device for Focusing Over a Large Depth," Journal of the Acoustical Society of America, No. 6, 1973, pp. 1628-30 and "Methods For Increasing The Lateral Resolution of B-Scan," Acoustical Holography, Vol. 5, 1973, pp. 391-413, and in an article by H. D. Collins entitled "Acoustical Interferometry Using Electronically Simulated Variable Reference And Multiple Path Techniques," Acoustical Holography, Vol. 6, 1975, pp. 597-619.
Certain imaging characteristics of conical axicons are analyzed in an article entitled "Imaging Properties of a Conic Axicon," Applied Optics, August 1974, pp. 1762-1764 by W. R. Edmonds. In an article entitled "Focal Depth of a Transmitting Axicon," Journal of the Optical Society of America, April 1973, pp. 445-449, J. W. Y. Lit, et al. examined the axial field distribution of conical and curved conical axicons for plane uniform incident energy and for gaussian distribution incident energy. Lit, et al. noted that the focal zone intensity distribution would vary from point to point along the axis of a conical axicon.
However, none of the foregoing references deal with the problems of employing an axicon in an optical playback system. The references suggest that something approaching a universal focusing system can be made using an axicon. However, they do not recognize the problems that the variation of intensity of energy focused onto the axis of an axicon can create in an optical recording and playback system. For example, with a conical axicon, the variation in focal zone intensity can be great enough to completely mask the modulation of a light beam scanned across an optical record.
U.S. Pat. No. 4,133,600 to Russell, et al. uses a conical axicon lens to try to alleviate tolerance requirements in regard to positioning or flatness of the record. However, the axicon is used only for formation of holographic lens means, not during recording or playback of data.
Likewise, none of the references suggest an axicon focusing system or method of making axicons which will eliminate or control this intensity variation. Accordingly, there remains a need for a focusing system which will provide both nearly universal focusing and a controllable intensity distribution within a focal zone for playing back data on an optical record.